Modern reactors used to convert heavy hydrocarbonaceous feedstocks such as petroleum residuum ("resid") to lighter, more valuable products typically employ slurry-type or ebullated bed hydroconversion processes. Both slurry-type and ebullated bed hydroconversion processes routinely are conducted in the presence of hydrogen addition rates ranging from 500 to 5000 standard cubic feet of hydrogen per barrel ("SCF/bbl") of reacted feedstock.
Slurry hydroconversion processes usually react heavy feedstock in the presence of hydrogen and a colloidally-dispersed or feedstock-soluble catalyst or catalyst precursor. In these conversion processes, a large fraction of the reactor's liquid contents usually is recycled within the reactor during operation to provide good mixing. If hydrogen and other gases are not substantially removed from the liquid recycle stream prior to the stream's reintroduction into the reactor's reaction zone, the recycled gases reduce conversion by occupying reactor volume that otherwise would be occupied by feedstock. Therefore, hydrogen and other gases must be separated from recycle liquids prior to their reintroduction into the reactor's reaction zone.
Ebullated bed hydroconversion processes react heavy feedstock in the presence of hydrogen and an expanded bed of supported catalyst. Ebullated bed processes are three phase processes in which gases must be separated from a primarily catalyst-free liquid which is recycled within the reactor to provide sufficient liquid velocity for catalyst bed expansion. Recycle streams in ebullated bed reactors also must be degassed to maximize conversion.
Reactor gas volume often is minimized by employing some type of liquid-gas separator located in the upper region of a resid hydroconversion reactor. The separator typically separates liquids and gases immediately prior to the separated liquids being recirculated downwardly into the reactor vessel. Locating the separator near the top, discharge end of the reactor minimizes the residence time of separated gases in the reactor. In an ebullated bed reactor, the liquid-gas separator typically is located below a vapor space and above a freeboard or catalyst-free region of the reactor which itself is located above the expanded catalyst bed. Locating the separator above the expanded bed region minimizes separator wear from catalyst impingement as well as other difficulties that result from trying to separate solids and liquids from process gases.
Liquid-gas separators suitable for use in ebullated bed and slurry-type resid hydroconversion reactors frequently include linear riser devices which transfer fluid from a liquid reactor region to the reactor's vapor space. The risers typically penetrate or are located adjacent to a liquid recycle pan or plate located at the upper end of a recirculation downcomer used to collect and recirculate degassed process liquids toward the bottom of the reactor. Our U.S. Pat. Nos. 4,804,458; 4,950,459; and 5,219,532 are representative of such systems. These patents disclose systems in which a liquid recycle pan having an outer diameter less than the inner diameter of the reactor is penetrated by one or more linear riser pipes used to transport gases collected below the recycle pan directly to a vapor space located above the recycle pan. U.S. Pat. No. 3,414,386 similarly discloses a liquid-gas separator in which riser pipes penetrate or extend upwardly from a liquid recycle pan or plate that is fixed or moveably sealed to a reactor wall.
Other types of liquid-gas separators used in slurry-type or ebullated bed reactors include hydroclones or other vortex-creating structures located within or external to the reactor. For example, U.S. Pat. Nos. 4,886,644 and 5,066,467 to Chan teach the use of a liquid-gas separator incorporating helical members located within riser pipes to separate gas from a liquid-gas mixture. These designs discharge gas upwards out the hydroclone while discharging liquid downwards into the reactor. U.S. Pat. Nos. 3,668,116; 4,012,314; and 4,354,852, on the other hand, teach the use of liquid-gas separators located external to a reactor vessel. While the use of hydroclones can, in some cases, provide adequate gas disengagement from process liquids, hydroclone structures having narrow passageways can tend to plug during operation, particularly with heavy feedstocks such as resid. External separators are not favored because they complicate reactor design and require fluid pathways outside the reactor which can waste heat and reduce active reactor volume.
While the foregoing patents provide for many liquid-gas separator designs, the considerable economic incentive provided by efficient liquid-gas separation in ebullated bed and slurry-type resid hydroconversion reactors demands novel, more efficient separator designs.